1. Field of the Invention
This invention relates generally to Doppler shifted radar systems for use in police traffic surveillance. More particularly, it relates to an improved transceiver design which is of a smaller size than those traditionally available and can be easily mounted on the rearview mirror of a patrol vehicle without impairing the operator's vision.
2. Description of the Prior Art
As is generally known, police traffic radar systems have been used heretofore in the prior art. The prior art traffic surveillance radar systems typically include a counting/display unit and a pair of antenna units (microwave transceivers) connected to the counting/display unit by serial data paths implemented through small flexible cables. The antenna units radiate microwave energy in the direction in which they are aimed. Frequently, the counter/display unit is mounted on top of the dashboard of the patrol vehicle. Also, it can be suspended from the radio rack underneath the dashboard by screws. One of the antenna units or microwave transceivers is generally mounted either to the front windshield, on top of the dashboard, or hooked adjacent the front fender of the patrol vehicle so as to point in the forward direction. The other antenna unit is generally mounted on the back of the patrol car such as on the rear windshield, headliner, or window trim.
Such conventional police radar systems in the current state of the art have been commercially available for many years and are typically exemplified by the police traffic radars manufactured and sold by Applied Concepts, Inc., located in Plano, Tex. Detailed descriptions and their operation have been described and illustrated in the following U.S. Pat. Nos.: 5,525,996; 5,563,603; 5,565,871; and 5,570,093 which are all assigned to Applied Concepts, Inc. and are incorporated herein by reference. There are also other known prior art traffic safety radars as evidenced by U.S. Pat. Nos. 5,504,488 and 5,528,246 which are both assigned to Kustom Signals, Inc. of Lenexa, Kans.
The problem with these prior art radar systems is that their design of the antenna unit or transceiver has used the combination of a microwave horn for collecting and concentrating the reflected microwave energy and a turnstile duplexer coupled to the horn via a circular antenna port. The turnstile functions to provide isolation between the transmitted and the received microwave energies in order to permit the transmitter and receiver circuitry to share the same antenna. While the microwave horn performs efficiently and cost effectively as a transmitting/receiving antenna, it suffers from the disadvantage of requiring a relatively large amount of space.
Even though the manufacturers have introduced the use of a dielectric lens at the aperture of the horn so as to reduce its length and still maintain maximum gain, there is created the problem that as the length of the horn is shortened the flair angle is increased. It has been found that the flair angle cannot be made much greater than 60.degree. in order to obtain a sufficient performance. Further, it has been empirically determined that an antenna with a 12.degree. beamwidth provides the optimal performance for police radars. Since the aperture diameter of the horn and the beamwidth of the antenna are related mathematically, this renders the Ka-band antenna having a 12.degree. beamwidth to have an aperture of a diameter slightly larger than 2 inches. Thus, this causes the length of the horn to be approximately 13/4 inches long. Further, due to the depth of the turnstile and its enclosure the overall length of the transceiver is increased. Typically, practical transceivers in common use are on the order of 3 inches in length which causes the transceiver to be quite large in size.
This large-size transceiver problem is further complicated by the fact that as of August, 1993, all passenger cars manufactured and sold in the U.S. must contain passive restraint systems (e.g., automatic seat belts and air bags) for both occupants of the front seat--driver and passenger, as mandated by the Federal Motor Vehicle Safety Standard #208. The requirement of air bags, especially air bags on the passenger side, have placed severe limitations on the location where the Doppler radar transceivers have been historically mounted in police vehicles. This is because the deployment of the passenger side air bag will require a considerable amount of space, thereby leaving very little room for placement of police equipment such as radar, video camera, shotgun and the like.
Since the air bags are inflated with an explosive force and will interact with any object in the deployment zone, automotive vehicle manufacturers have generally recommended that this deployment zone be kept free of any equipment in order to reduce the risk of injury to the vehicle occupant or the vehicle itself. At present, the air bag on the passenger side cannot be legally disabled, but this law may be altered in the future. The problem of location of the radar system caused by the necessity of air bags is further exacerbated by the constraint that the transceiver of the radar system must be positioned so that the operator's vision is not impaired. Moreover, since the head of the transceiver is susceptible to random noise interference from various other equipment contained in the control vehicle such as the vehicle's defroster fan, this further restricts the area that can be used to position the transceiver.
While radar manufacturers have heretofore addressed the large-size problem of radar systems by reducing the signal processing circuitry inside of the counter unit and the electrical components used in the display unit, the inventors are unaware of any developments which allow for a substantial reduction in the size of the "classical" transceiver design. However, there have been attempts made in the prior art to design the microwave transceiver to operate from X or K-band frequencies to a Ka-band frequency since the transmitting of higher frequencies will decrease proportionally the size of the transceiver. The improved transceiver construction of the present invention has a volumetric reduction on the order of 50% when compared with the conventional transceivers of the prior art. Further, the transceiver of the present invention is a fully integrated structure which can be manufactured economically and can be easily mounted behind the rearview mirror of a patrol vehicle (which is an air bag safety zone) without hindering use of the rearview mirror.